Zinc phosphide (Zn3P2) is a promising photovoltaic absorber composed of earth-abundant and non-toxic elements, with a near-optimal bandgap for solar energy conversion. Its practical implementation is limited by a complex defect landscape and the difficulty of growing high-quality thin films on conventional substrates due to lattice and thermal expansion mismatch. Quasi-van der Waals epitaxy on graphene offers a route to overcome these constraints by decoupling the epilayer from the substrate, but introduces new challenges related to nucleation, microstructure, and defect formation.
This thesis investigates the relationship between growth conditions, resulting microstructure, and the optoelectronic properties of Zn3P2 thin films grown on graphene. By comparing graphene substrates of different quality, it is shown that nucleation is predominantly governed by surface topography in high-quality graphene. Both MBE and MOVPE yield films with a well-defined out-of-plane orientation imposed by the graphene substrate and random in-plane alignment, resulting in polycrystalline layers. MOVPE significantly reduces the nucleation density compared to MBE, enabling grain sizes approaching 10 um and thereby reducing the density of grain boundaries.
Cathodoluminescence measurements reveal that the emission is dominated by sub-bandgap recombination, most likely associated with band-tail states, with spatial variations correlated to microstructural features such as antiphase boundaries and grain boundaries. In MBE-grown films, additional highly localised emission at grain boundaries is attributed to strain-induced states, which are absent in MOVPE-grown films, indicating improved material quality. Grain boundaries are identified as efficient non-radiative recombination centres, with characteristic recovery lengths comparable to the grain size, highlighting the critical importance of grain-boundary passivation for device applications.
Electrical transport measurements show thermally activated behaviour and low mobilities, with little dependence on temperature, composition or growth conditions, indicating that charge transport is dominated by microstructural limitations, in particular grain boundaries and defect states.
Overall, this work demonstrates that the optoelectronic properties of Zn3P2 grown by quasi-van der Waals epitaxy on graphene are primarily governed by microstructure. While MOVPE improves structural quality through reduced nucleation density, further progress towards photovoltaic applications will require control over grain size and defect formation, as well as effective strategies for grain-boundary passivation.
